In integrated circuit (IC) photolithography technology, significant effort has been expended on increasing the resolution of photoresist processes because greater resolution enables a greater number of circuits to be placed on a single chip. This increase in circuit density increases the potential complexity and speed of the resulting integrated circuit. In these photoresist processes, a photoresist is spun onto a wafer being processed and then selected regions of the photoresist are exposed by light that passes through a projection mask. The pattern of the projection mask determines which regions of the photoresist are exposed by this light. The photoresist is then developed to produce a contact mask on the wafer. The quality of the integrated circuit features is strongly affected by the quality of the photoresist contact masks used in the various IC processing steps. Therefore, it is very important that this contact mask accurately duplicate the pattern of the projection mask.
Present techniques in optical projection printing can resolve 1 micron lines in photoresist with good linewidth control when flat, low reflectivity substrates are used. However, when exposing photoresist on substrates with surface topography, there are photoresist control problems introduced by optical reflections and by photoresist thickness nonuniformity. Light reflected from the photoresist substrate interface produces optical interference with the incident light, producing undesired variation in the light intensity within the photoresist. This undesired variation of photoresist exposure degrades the IC features generated with this photoresist contact mask.
Linewidth control problems also arise from substrate topography. Because of the small depth of field of photolithography exposure systems, the photoresist film should be thin and planar. However, in the later steps of these photoresist processes, the photoresist is deposited on a surface that is nonplanar because of features produced in previous processing steps. Such nonplanarity will produce thickness variations of the photoresist layer and can also degrade the sharpness of imaging onto this layer because of the limited depth of field.
In U.S. Pat. No. 4,362,809 entitled "Multilayer Photoresist Process Utilizing an Absorbant Dye" issued to Chen et al on Dec. 7, 1982, these problems are overcome by a two layer process in which the bottom photoresist layer is thick enough to produce a planar top surface. This planarizing photoresist layer also contains a dye used to absorb incident light before it has a chance to interfere with the light in the thin top photoresist layer. The planarization by the bottom layer enables accurate exposure of the top photoresist layer. The dye in the bottom layer eliminates interference that would otherwise occur by light reflected by the substrate-bottom photoresist layer.
In U.S. Pat. No. 4,705,729 entitled "Method for photochemically enhancing resolution in photolithography processes" issued to James R. Sheats on Nov. 10, 1987, in place of this two layer photoresist process is a process utilizing a thick planarizing photoresist layer on top of which is formed a thin layer containing a bleachable dye. Light incident on this layer through a projection mask bleaches selected portions of this layer thereby creating a contact mask for use in exposing the bottom photoresist layer. While the top layer is photobleached, the underlying photoresist layer is made to be substantially unaffected by the photobleaching process. The further bleaching of the top layer is then prevented during exposure of the bottom layer through this contact mask.
In one embodiment of this process, the top layer is bleached at a high rate only if the light intensity is above a certain threshold level, called the reciprocity level. Below this level, the amount of reaction is a function of the light dosage (i.e., the intensity times the exposure time). Above the reciprocity threshold, the rate of bleaching is much faster and is a monotonically increasing function of intensity. Therefore, during the bleaching process, the intensity is high and the total dose of light is selected to bleach the top layer without significantly exposing the bottom photoresist layer. Then the bottom layer is exposed through this contact mask with uniform light of intensity lower than this threshold level, thereby substantially avoiding further bleaching of the bleachable layer.
In a second embodiment, the top layer bleaches only if oxygen is present in the ambient atmosphere. Therefore, during exposure of the photoresist layer through this contact mask, oxygen is excluded from the surface of this contact mask.
Unfortunately, the material utilized for the thin bleachable layer was far from optimal. In the first embodiment, the thin bleachable layer is poly(methylmethacrylate) (PMMA) containing approximately 20 wt. % acridine. This choice has two drawbacks. The intensity threshold at which bleaching occurs is sufficiently great that an excimer laser is needed to supply this intensity of light. Such exposure systems are more expensive than conventional systems and the high intensity of light can damage the optics, necessitating their more frequent replacement.
In the second embodiment, (presented in U.S. Pat. No. 4,705,729, which is assigned to the same assignee as the present application), anthracene derivatives are utilized in a polymer to form the bleachable layer. This embodiment utilizes low intensity light so that an excimer laser system is not required. However, most polymers (which must be present as a medium in which the dye is dissolved) are not very permeable to oxygen. Thus, very long exposure times are required in order to allow oxygen to diffuse into the film in sufficient quantities, otherwise a detrimental reciprocity failure occurs. For example, poly(ethylmethacrylate) and poly(styrene) are commonly available, good film-forming polymers into which anthracenes may be dissolved in large quantities (15-40 wt. %), but if they are used as the matrix to hold the bleachable dye, then exposure times on the order of 10 to 20 seconds are needed. This is too slow for a high-throughput manufacturing plant which requires an exposure time on the order of or less than 1.5 seconds and preferably less than 0.5 seconds.